1. Field of the Invention
The present invention relates to a connection structure of an inkjet recording head, and particularly to a structure of a connecting terminal which is formed on a surface of an actuator of an inkjet recording head to be connected to a wiring board from which electric power is supplied.
2. Discussion of Related Art
There is disclosed in JP-A-2003-159795 corresponding to U.S. Patent Application Publication No. 2003/063449 A1, for instance, an inkjet recording head of drop-on-demand type constructed such that a cavity unit constituted by a laminar structure consisting of plural plates is connected to a piezoelectric actuator which has two rows of pressure chambers formed inside, and activation portions (energy generators) corresponding to the respective pressure chambers. To enable to apply voltage to the activation portions, surface electrodes as connecting terminals corresponding to the respective activation portions are formed on an upper surface of the piezoelectric actuator along two margins extending in a longitudinal direction of the upper surface, and connecting electrodes of a flexible flat cable which is provided to transmit control signals from an external device are superposed on, and connected to, the surface electrodes (connecting terminals) of the piezoelectric actuator.
It has been proposed, as disclosed in JP-A-11-147311, that the flexible flat cable be constituted by a laminar structure of substrates respectively having wiring on one of their opposite surfaces, and openings be formed through at least one of the substrates so that wiring on another substrate disposed on the at least one substrate is exposed to the outer space through the openings, at each of which a bump electrode (as a connecting electrode) is formed to be connected to a corresponding one of the surface electrodes on the piezoelectric actuator.
Such bump electrodes are generally formed of a solder alloy, which is softened or melted by application of heat.
On the other hand, the piezoelectric actuator is generally formed by laminating three green sheets of ceramic material, namely, a first green sheet, a second green sheet, and a third green sheet as a top plate, and then firing the laminate. The first green sheet is made of a ceramic material as a piezoelectric material on which a pattern of individual electrodes is formed. The second green sheet is similar to the first green sheet but a pattern of ‘common’ electrodes is formed thereon instead of the individual electrodes, and the green sheet as the top plate is similar to the first and second green sheets and has surface electrodes formed thereon. The individual and common electrodes and the surface electrodes, which are electrically connected to the individual and common electrodes, are formed on the respective green sheets by screen printing with an Ag—Pd (silver-palladium)-based paste which is electrically conductive.
Although such a kind of the surface electrodes electrically connected to the individual and common electrodes is excellent in its wettability with a solder alloy, it suffers from low bonding strength with an upper surface of the piezoelectric actuator due to a small thickness of the surface electrodes, leading to the following drawback.
Since the flexible flat cable generally comprises a flexible substrate of a synthetic resin, a degree of its thermal expansion/contraction is relatively large. Therefore when the flexible flat cable is repeatedly used for a long term under conditions where the temperature of the flexible flat cable varies greatly, the distance between each adjacent two of the bump electrodes of the flexible flat cable increases and decreases, leading to peel-off of the surface electrodes as connected to the bump electrodes from the upper surface of the piezoelectric actuator, at a part where the bonding strength is relatively low. Thus, electric disconnection often occurs.
To solve this drawback, the present applicant has proposed to form eternal electrodes on respective surface electrodes.
It was revealed that the bonding strength between the solder alloy or the bump electrodes and the external electrodes can be enhanced by this arrangement. However, the problem of occurrence of electric disconnections has not been solved by this arrangement, since a fillet, which is formed such that when a molten solder alloy flows from a surface of the external electrode over a surface of the surface electrode and is solidified there, cracks upon expansion and contraction (especially, contraction) of the flexible flat cable caused by the variation or change in the temperature of the flexible flat cable.